1. Field of the Invention
The present invention relates to a process for forming a superconducting film. More particularly, the present invention relates to a process comprising forming a plurality of metal layers (buffer layers) on a ceramic substrate and forming a wiring pattern of an oxide superconductor on the buffer layers.
2. Description of the Related Art
As methods of forming a wiring pattern on a ceramic substrate for the circuit board (to interconnect between electronic elements) to be used for an electronic apparatus, there can be mentioned a method comprising forming a thin film on the substrate by vacuum deposition or the like and forming a wiring pattern by the etching technique, and a method comprising forming a wiring pattern on the substrate by using a paste composed mainly of a wiring conductor, by the printing technique, and firing the printed pattern to form a thick film pattern. As the material of the film, there can be mentioned not only normal conductors such as Cu and Mo but also superconductors such as Nb, Y--Ba--Cu--O type (Ref. M. K. Wu, L. R. Ashburn, C. J. Trong, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu: Phy. Rev. Lett., 58, (1987) 908) and Bi--Sr--Ca--Cu--O type discovered by H. Maeda et al. (Ref. H. Maeda, Y. Tanaka, M. Fukutomi, and T. Asano: Jpn. J. Appl. Phys. 27 (1988) L209) and Bi--Pb--Sr--Ca--Cu--O was reported by M. Takano et al. (Ref. M. Takano, J. Takada, K. Oda, H. Kitaguchi, Y. Miura, Y. Ikeda; Y. Tomii, and H. Mazaki: Jpn. Appln. Phys: 27(1988) L 1041). Nevertheless, when a film of a conductor such as the above-mentioned normal conductor or superconductor is formed on the substrate, the intended substance does not have a homogeneous composition, due to a mutual reaction between the film and substrate, for example, a chemical reaction, and thus sometimes the intended performance, especially of the superconductor, is not obtained.
A superconducting film device resembling the superconducting film prepared according to the process of the present invention is known (Japanese Unexamined Patent Publication No. 63-279521), but this device is prepared by forming a metal film having a thermal expansion coefficient of at least 10.times.10.sup.-6 /K in both directions on a ceramic substrate, and forming a covering film of a perovskite type metal oxide super-conductor. This known technique is different from the process of the present invention in that, in this known technique, to prevent a formation of cracks in the coating film of the superconductor, a conductor layer of a metal having a specific thermal expansion coefficient is interposed between the ceramic substrate and the superconductor layer.
Japanese Unexamined Patent Publication (Kokai) No. 63-305574 (Publication Date: Dec. 13, 1988) discloses a superconducting substrate for forming a superconductor on a supporting substrate comprising a composite substrate with a stabilizing material which does not react chemically, interposed between the supporting substrate and the superconductor.
Japanese Unexamined Patent Publication No. 63-305574 also discloses the formation of a superconducting film of the Y--Ba--Cu--O system, which comprises forming the metal (Pt, Pd, Ag, Au) layer by a deposition, sputtering, flame spaying or plating process, and the like, on the Al.sub.2 O.sub.3 substrate, and forming the superconducting film on thus-obtained metal layer. There is no disclosure or suggestion therein that the double layer of the above metal layer is formed as a stabilizing material.
Therefore, this conventional technique is clearly different from the present invention in object and structure.